Thermoplastic sheet material is conventionally formed into predetermined contoured configurations for use as the housings for illuminated signs, the shells for whirlpool and bath tubs, and a wide variety of other products. The formation of such products from plastic sheet material is typically achieved by a heating and molding process for which a number of apparatus are presently available. Basically, these apparatus include a heating oven and a table on which the appropriate mold is supported and a frame for engaging the plastic sheet peripherally. One or both of the table and the heating oven are movable with respect to the other for selective disposition of the table outside the oven for loading and within the oven for heating and molding of the plastic sheet material. Relative movement between the sheet engaging frame and the mold is also usually provided for disposition thereof apart from one another during heating of the plastic sheet while within the oven and for movement of the frame and mold toward one another once the sheet reaches a plasticized state for applying the sheet to the mold surface to be conformed to its contour. Most such apparatus additionally provide for the application of vacuum through the mold to the plastic sheet to draw the sheet into complete conformity with the mold surface. Typical examples of the above-described apparatus and methods of forming thermoplastic sheet material are disclosed in U.S. Pat. Nos. 3,553,784; 4,018,551; 4,099,901; and 4,170,449.
Such apparatus and methods have proved satisfactory for the most part for their intended purposes. However, problems do sometimes arise in suitably controlling the degree of heating of the plastic sheet material performed by these apparatus and methods. No manner of thermostatic control of the temperature produced within the ovens to which the sheet material is heated is known to be employed in any of the conventional apparatus and methods of the type described. The apparatus and methods are routinely operated for forming plastic sheets of a variety of different thermoplastic materials, as well as differing sizes, thicknesses and colors of sheets of all such materials. It is a particular concern in the heating of the plastic sheets utilized that the sheet be heated only to the extent necessary to achieve complete plasticization across the full extent of its lateral dimensions and its thickness but without overheating any portion of the sheet material to avoid burning, tearing, separation, over stretching or other undesired deformation of the plastic material out of a generally sheet-like form. As will be understood, the aforementioned variety of sheets utilized in apparatus and methods of the present type necessitate differing degrees of heating of the sheets. Since no thermostatic heater control is provided, the needed control of the heating of the sheet material is conventionally accomplished only by adjusting the time of heating in relation to the pertinent sheet characteristics determining the rate and degree of heating thereof, i.e. the material composition, size, thickness and color of the sheet. Disadvantageously, the lack of thermostatic heater control causes the heating elements of the oven to generate heat at progressively increasing temperatures over the time of energization of the oven. Accordingly, in certain circumstances, particularly when an extended time of heating is required under the conventional practice, the heating control provided by regulation of the heating time may be entirely unsatisfactory in preventing overheating and resultant damage to the sheet material since the temperature in the oven may reach an undesirably high degree because of the extended heating time. It is typical in some apparatus and methods that the heating ovens utilized may cause uneven heating of the sheet material at different areas thereof which only serves to accentuate and compound the above-described problems.